1. Field of the Invention
The present invention relates to a sheet, a plate, a bar or a wire which is made of titanium and has high ductility and low material anisotropy, and to a method of producing the same, and, in particular, to a hot-rolled or cold-rolled sheet, a hot-rolled or cold-rolled plate, a hot-rolled or cold-rolled bar, or a hot-rolled or cold-rolled wire, which cold-rolled bar or wire includes that subjected to drawing, each of which is made of commercially pure titanium which is classified as the second or third category in JIS (JIS class 2 or 3), or low-alloyed titanium where a small amount of Fe is added, and a method of producing the same.
2. Description of the Related Art
Titanium has the properties of low weight, high strength, high corrosion resistance and the like, and has been used in the fields of aviation, chemistry, maritime engineering, electric power generation, etc. in which these properties have been required.
Titanium is classified into two categories, namely alloyed titanium and commercially pure titanium. of the two, commercially pure titanium has the properties of medium strength, excellent corrosion resistance, relatively excellent workability and relatively good weldability, and it has been used for producing various shapes of products including a heavy or medium plate, a hot-rolled or cold-rolled strip, a sheet or a plate cut out therefrom, a welded pipe produced by forming and welding a sheet, a large diameter straight round bar, a rectangular straight bar, a bar or wire coil, a medium-to-small diameter bar or wire cut out therefrom, a seamless tube formed by hot extrusion, and the like.
A sheet or a plate, a bar or a wire made of commercially pure titanium is classified into any one of the first to fourth categories under JIS (JIS class 1 to 4) based on added elements and strength, and, in case of JIS class 2 which is most commonly employed, it is specified that the oxygen content is 0.20 mass % or less, the nitrogen content is 0.05 mass % or less and the Fe content is 0.25 mass % or less. In case of JIS class 3 which represents higher strength than JIS class 2, it is specified that the oxygen content is 0.30 mass % or less, the nitrogen content is 0.07 mass % or less and the Fe content is 0.30 mass % or less. (Hereafter, the amount of each chemical component is expressed in terms of mass %.)
In reality, however, the commercially pure titanium of JIS class 2 or 3 contains nitrogen at 0.015% at most and Fe at 0.1% at most, and thus it has literally been pure titanium except that oxygen at 0.07 to 0.3% and unavoidable impurities have been contained.
A sheet or a plate, a bar or a wire made of commercially pure titanium of JIS class 2 or 3 is used for producing products having various sectional shapes and dimensions and widely used in many fields, as stated above. Therefore, in order for a titanium material to be secondary-formed into a complicated shape by bending, cold forging or flat rolling (a bar or a wire having a round section is cold-rolled into the shape of a flat sheet or plate), a titanium material which can secure a higher ductility and a higher cold workability without the strength deteriorating has been strongly required.
In case of high strength titanium alloy, as a method of obtaining both high strength and high ductility, there is a method of adding both Fe and nitrogen at the same time as disclosed in Japanese Unexamined Patent Publication No. H8-833292 (International Publication No. WO96/33292). It is estimated that this method has a possibility of being applied to a titanium material with a strength level corresponding to JIS class 2 or 3 in which the strength is rather low. However, when the addition amount of Fe increases, in case of a titanium sheet or plate, the material anisotropy in a sheet or a plate plane becomes large, and, the problem here has been that, though the material shows an excellent strength and an excellent ductility in the longitudinal direction (L direction), it shows too high strength and thus low ductility in the directions perpendicular to the L direction. Furthermore, in case of a titanium bar or wire, the material anisotropy in a sectional plane becomes large, and, the problem here has been that, though the material shows an excellent strength and an excellent ductility in the longitudinal direction (L direction), it shows too high strength and thus low ductility in the directions perpendicular to the L direction, namely, in the circumferential and radial directions.
The material anisotropy stated above is mitigated by changing the rolling directions intermediately, namely by adopting a so-called cross rolling. However, the drawbacks are that the method cannot be applied to a lengthy material such as a strip of which a high productivity is required because of the dimensional restrictions of a rolling facility and that, even in case of a relatively short material such as a plate, the production cost increases. Besides, the aforementioned cross rolling cannot be applied to a bar or a wire.
In case of a wire having a very fine diameter, though there is no problem in terms of material properties as long as the property merely in the L direction is excellent, as, in general, the wire is subjected to various working not only in the L direction but also in the circumferential and radial directions during secondary working, it is required to sufficiently secure the workability in these directions too.